Welding rusty metal without cleaning it is a situation that you will face sooner or later as a new welder. For instance, you want to do a repair, but you don’t have the proper cleaning equipment at hand.
In this article, you can read about the problems when welding rusty metals and the best options.
Table of Contents
- Can you weld rusty metals?
- What are the problems when you weld rusted metals?
- Is welding rusty metal bad for you?
- What is the best way to weld rusty metal?
- Can you weld rusty metal with flux-cored wire?
- Can you MIG weld rusty metal?
- Can you TIG weld rusty metal?
- Can you weld rusty metals with oxy fuel?
- Can you weld rusty thin metal?
Can you weld rusty metals?
You can weld rusty metals if you have the proper welding process and consumables for the job at hand. However, you cannot weld rusty metal for critical work because rust lowers the tensile strength and causes many defects. Furthermore, you want to take extra precautions to protect yourself from increased fumes.
As a safety practice, you must not do any critical work with rusted or any kind of dirty metals. For example, if a welded joint breaks and people get hurt.
Also, any welding with commercial intent needs to meet the service requirements of the project. The welds must hold as intended for the long run, not only when they are in the workshop.
However, for quick repairs or small joints that don’t need to meet the strength of the base material, welding over rust is a common practice. You can weld through light or even thick rust up to a point.
Before you start welding rusted metal, you must clean it as best as possible, depending on your situation. So even if you won’t reach bare metal, you will have the least amount of impurities in the bead. This way, you will get stronger welds with fewer defects.
What are the problems when you weld rusted metals?
Now let’s see the effects of rust on welding in more detail.
1. Lower tensile strength
Rust is an iron oxide, meaning it contains oxygen. The arc will break down a part of these oxides. Free oxygen will react with alloying elements such as manganese and silicon in the weld metal. The reduction of these elements lowers the joint’s tensile strength.
2. Lack of fusion
Because rust acts as a heat barrier, it worsens the lack of fusion defect. Lack of fusion is when the weld metal doesn’t mix with the base metal but sits on top. Lack of fusion is dangerous because it weakens the joint and isn’t detectable to the untrained eye.
3. Poor electrical flow
Rust will prevent the electricity’s normal flow between the working (or grounding) clamp and the electrode. As a result, you are going to have trouble starting the arc. If the rust is too thick, the arc might not start at all.
4. Unstable arc and spatter
When you weld, the impurities will give you an unstable arc, resulting in increased sparks and spatter. Furthermore, it would feel like you are welding with lower amperage.
5. Poor weld metal fluidity
Rust will also restrict the puddle’s flow making it stiffer and slower.
As a result, the bead will be narrow, asymmetrical, convex, with undercuts since the sides will not fill with weld metal.
To improve a stiff puddle, you can weld high a higher amperage or at a slower speed.
6. Porosity and inclusions
As mentioned, rust will release oxygen in the weld metal. Oxygen will create porosity and will combine with various elements creating many inclusions.
Porosity and inclusions make the bead less dense and act as mechanical and corrosion stress points.
As a result, the joint might break under high loads, and if under a corrosive environment, it will develop rust at higher rates.
Is welding rusty metal bad for you?
Pure rust can worsen the fume generation problem because it makes the arc unstable. But things can get worse if there are paint or grease leftovers mixed with the rusted metal. The resulting fumes will be more complex thus more harmful.
You must always keep your head away from the fume flow and wear a welding respirator for added protection.
Furthermore, if you try to remove some of the rust, you must avoid inhaling the dust you will create.
An N95 respirator or filter is enough for rusted metal. Still, you need higher protection if there are paint or oil residues in the rust.
Weldpundit has an article on welding respirators for home welders to learn more about them.
In the second part of the article, we’ll see how each welding process can handle rusted metals and the best consumables to use.
The welding industry understands that cleaning the workpieces to perfection is not always practical, necessary, or cost-effective.
Manufacturers provide various rods and wires designed to give better results when you weld over rust, mill scale, and other surface contaminants.
The best consumables for rusty metals have flux material that forms slag capable of trapping impurities in the puddle.
All the rods and wires described below are for low-carbon steel.
What is the best way to weld rusty metal?
The first welding process that comes to mind when considering welding over rust is stick welding (SMAW).
Stick welding is the best welding process for rusty metals because it uses covered welding rods. These rods create an aggressive arc that can burn the rust. Furthermore, the flux covering creates a slag material that cleans the puddle. Stick welding is excellent for quick repair work on rusted metals since it’s very portable.
Stick welding electrodes have a wire core and a flux coating. The flux coating creates slag with high amounts of deoxidizers and other scavengers that trap most pollutants and drive them to the surface.
Another significant advantage of stick welding is that it has rods with cellulosic flux material.
What is the best welding rod to use on rusty metal?
The best welding rod for rusty metal is the E6010 and, after that, the E6011. These rods have a cellulosic-based coating that creates an aggressive arc that can burn thick rust, find the metal underneath and weld it.
Cellulose is an organic material (fibers) that release gases such as carbon dioxide (CO2) and hydrogen. These gases create a hot and aggressive arc.
Furthermore, the gas coverage is wide, and you can manipulate the rod back and forth. We call this the whip and pause technique and helps to burn the rust and preheat the metal before the puddle arrives.
As a result, cellulosic rods can weld heavily rusted metal. They are excellent for quick repairs on exposed farm equipment, etc.
The E6010 is the strongest against rust and runs on direct current (DC+). But it cannot work with most common welding machines. You don’t want to buy a box of E6010s and discover later that they cannot even start.
As a great alternative to weld over rust, you can use the E6011, which can also run on alternating current (AC). If your welder can burn an E7018, it can burn an E6011 as well.
Rutile stick rods such as the E6013 and the E7014 can also weld on rusted metals.
These electrodes will hold the pieces together but are not as efficient as the cellulose electrodes. They cannot weld thick rust.
It is also possible to weld rusted metals with low-hydrogen electrodes, such as the E7018.
This rod has a basic flux covering that can capture excessive oxygen and prevent the creation of inclusions. As a result, the bead is stronger and more ductile.
However, an E7018 doesn’t create a strong arc and cannot weld thick rust.
One disadvantage of stick welding, especially with cellulosic rods, is that it cannot weld thin rusted metals.
Always prefer the DC+ polarity since it’s the best for deeper penetration.
Can you weld rusty metal with flux-cored wire?
You can weld rusted metal with a flux-cored wire and receive good results because it contains flux material. The flux creates an aggressive arc and contains elements that can clean the puddle. It is the next best welding process for rusted metals after stick welding.
Flux-cored welding uses electrodes in the form of wires in spools. The wire is tubular with a core filled with flux material.
Similar to a stick electrode, the flux contains a lot of deoxidizers and produces slag. It’s like a stick electrode turned inside out that offers faster welding speeds and it’s easier to use.
There are two types of flux-cored wires, the self-shielded and the gas-shielded (or dual-shield).
Self-shielded wires for rusted metal
Self-shielded wires are the most common, and they don’t need external gas shielding. They are very good at welding thick layers of rust. However, they cannot compete with the effectiveness of cellulosic stick welding rods.
The popular general-purpose wire E71T-11 can offer great results on rusty metals because it has large amounts of deoxidizers.
If you need the best cleaning efficiency, then go for the E71T-14 wire. This is because it has the highest amounts of silicon and manganese.
But keep in mind that E71T-14 is a wire for single-pass welds and is hard to find.
These wires work only with DC- polarity.
Gas-shielded wires for rusted metals
Gas-shielded wires produce slag but need external shielding gas to protect the puddle. They are not as efficient for rusty metals as the self-shielded wires, but can still offer far better results than solid MIG wires.
The most popular wire of this type is the E71T-1, which has enough deoxidizers to weld over light rust.
A better wire for rusty metal is the E71T-2, but it’s a single-pass wire and available on thicker wires and larger spools.
For puddle protection, these wires need 75% argon and 25% CO2 (75/25 or C25) or 100% CO2 shielding gas.
But, dual-shield wires are costly, hard to find in small spools, and need a high amperage 240V welding machine. They are suitable for high production work.
Can you MIG weld rusty metal?
MIG welding (GMAW) can weld over thin layers of rust, but it’s not as good as stick or flux-cored welding. The reason is that the MIG wires don’t have flux material to produce slag and have much lower amounts of deoxidizers.
MIG wires try to offer good welding results when welding rusty metals by containing silicon and manganese. These elements act as deoxidizers. Furthermore, silicon enhances the puddle’s fluidity and counteracts the stiffness caused by rust.
As for shielding gases, a 100% CO2 gas will provide better penetration and material fusion.
A 75/25 mix is better for thinner metals and also creates less spatter.
MIG wires for rusted metals
The ER70S-6 can weld over thin layers of rust because it has the highest silicon and second-highest manganese amounts (after ER70S-7, but this one has specific uses). It is the most popular and versatile MIG wire and comes in many diameters and spool weights.
The ER70S-2 wire contains special deoxidizers such as zirconium, titanium, and aluminum. This makes it better to weld over rust, but it’s much more expensive and harder to find.
The ER70S-3 wire has low amounts of deoxidizers and requires clean metal.
A serious consideration on MIG welding over rust
The common MIG welding machines (under 200A) use the short-circuit mode to transfer the weld metal. This is a low-heat mode with low penetration. As a result, it’s not very efficient against the rust-related problems described earlier, especially the lack of fusion defect.
MIG welding with low heat on rusty metal can lead to a superficial joint that will fail on the first sign of mechanical or thermal stress.
One tip to prevent lack of fusion is to always point the arc in front of the puddle. The following image shows the V pattern that offers better results than the circular one.
The other commonly used transfer mode is the spray mode and isn’t affected by increased lack of fusion problems.
The spray more is far better to weld over rust, but you need a high-amperage machine to use it.
Can you TIG weld rusty metal?
TIG welding (GTAW) will never offer good results when welding rusty metals. Even, if you do a quick repair on superficial rust, you will have an unstable arc, porosity, and a contaminated tungsten electrode to grind.
TIG welding rods and solid MIG wires are the same but in different shapes. So the most recommended TIG rods are the ER70S-6 and the ER70S-2.
Can you weld rusty metals with oxy fuel?
You can weld rusty metals with oxy fuel, but since you already have the equipment, you can use it to clean the rust. You can use the technique called flame cleaning.
You set a neutral flame, then pass it over the surface by using a small angle, almost parallel to the surface.
The flame will burn the rust and leave a clean enough surface to start welding.
But be careful when using flame cleaning. If the metal’s surface has grease or paint, it might ignite if there is an excess of oxygen in the flame.
The RG60 oxy-fuel rod contains almost double the amount of manganese and silicon than RG45 and is more suitable for rusted metals.
Can you weld rusty thin metal?
Welding rusty thin metal is exceptionally hard to do. That’s because rust digs into thin metal, making it even thinner and asymmetrical. Even with the best low-heat practices, it’s easy to burn through the damaged workpiece. The best thing to do is replace the rusted part.
The best process for thin rusted metal is MIG welding. Still, if the metal is thin sheet metal and the rust extensive, the only result would be cutting the corroded metal instead of welding it. That’s because you will not find solid metal to fuse it with the wire.
TIG is excellent for thin metal but has trouble if it’s rusted.
Stick welding and flux-cored wires cannot weld very thin metals even if they are rust-free because they create an aggressive arc.
If you have a stick welder, you can try a small diameter E6013 rod since it is the best to weld thin metals.
If you consider welding rusty car panels, watch the following video.
It is always best to clean the rust next to the joint. A piece of coarse sandpaper or a simple wire brush can remove a lot of rust.
However, there are several cases when you can’t or don’t want to clean the metal correctly.
You can weld over rust by selecting the appropriate process and consumables. However, ensure the weld is strong enough for the job.
The best welding process to weld over rust is stick welding with cellulosic electrodes. After that, flux-cored welding with self-shielded wires.
MIG welding can weld over light rust. But keep in mind the lack of fusion defect. TIG welding is not suitable for welding rusty metals.
Finally, with oxy-fuel welding, you can clean the surface of rusty metals and weld on clean metal.
Increasing the amperage and/or welding at a slow speed can improve the results.
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